1. Field of the Invention
The present invention relates to disk drives. In particular, the present invention relates to a disk that employs multiple index marks.
2. Description of the Prior Art
As shown in FIG. 1, a conventional disk drive employs at least one disk 2 having a plurality of concentric, radially spaced tracks 4. Each track 4 comprises a number of data sectors interrupted by a number of embedded servo sectors 6 used to position a head over a target track and maintain the head over a centerline of the target track during read/write operations. Typically, both sides of the disk 2 are used to record information wherein a respective head is actuated radially over each disk surface. When the disk drive is powered on or a head switch occurs, the embedded servo sectors 6 are located and then used to servo the respective head over the disk 2.
Each servo sector 6 comprises a servo address mark (SAM) 8, a preamble 10, a sync mark 12, an index mark 14, servo data 16, and servo bursts 18. The SAM 8 comprises a predetermined sequence of bits (e.g., a plurality of “0” bits), which can be detected asynchronously during a power on or head switch event. Once the SAM 8 is detected in one of the servo sectors 6, the location of the remaining servo sectors 6 can be determined and suitable timing circuitry is configured to enable demodulation circuitry at the appropriate time (synchronous with the occurrence of each servo sector). The demodulation circuitry processes the preamble 10 to synchronize timing and gain control circuitry, and the sync mark 12 identifies the beginning of the servo data 16. The servo data 14 comprises information about the track, such as a track ID used during seek operations. Each servo burst 18 following the servo data comprises a “burst” of magnetic transitions, which are typically integrated to generate a servo burst signal. The servo burst signals are evaluated relative to one another to generate a position error signal (PES) used to maintain the head over a centerline of the track (tracking) during read/write operations.
Before performing a write operation to a particular track, it is important to accurately locate the target data sector to prevent inadvertently overwriting data in another data sector. To this end, prior art disk drives have employed a disk format wherein each data sector comprises a sector ID used to identify the sector. However, recording a sector ID in each data sector reduces the overall storage capacity due to format inefficiencies. More recent disk drives achieve improvement in format efficiency through an “ID-Less” disk format wherein the location of each data sector is derived from the servo sectors 6. One technique for identifying the data sectors in a track is to consider one of the servo sectors 6 as an INDEX servo sector as shown in FIG. 1. The INDEX servo sector records a valid index mark value (e.g., a “1” bit) in the index field 14, and the remaining servo sectors 6 record a non-index mark value (e.g., a “0” bit) in the index field 14. Once the INDEX servo sector is located, the remaining servo sectors 6 are referenced incrementally (e.g., using a servo sector counter). The data sectors are then identified using suitable timing circuitry after the occurrence of each servo sector 6. However, if the servo system loses synchronization after initially detecting the INDEX servo sector, the error condition is not detected until the INDEX servo sector is detected again. In addition, when the disk drive switches heads in order to access a different disk surface, it may require up to an entire revolution of latency to detect the INDEX servo sector. Still further, if a single bit is used to identify the INDEX servo sector, it may require several additional revolutions to ensure that the INDEX servo sector is not falsely detected due to noise.
U.S. Pat. No. 5,768,044 discloses a method for reducing the rotational latency after a head switch by adjusting the servo sector counter to account for the skew between the servo sectors on each disk surface. However, this technique also encodes a servo sector number into each servo sector to increase reliability in the event that the servo sector counter is adjusted by the wrong amount. Recording a servo sector number into each servo sector is undesirable because it reduces the format efficiency similar to recording a sector ID in each data sector.
There is, therefore, a need to increase servo synchronization robustness as well as reduce the rotational latency during a head switch in a disk drive employing ID-Less data sectors without reducing the format efficiency of the embedded servo sectors.